There have been heretofore proposed a variety of methods of removing sulfur dioxide, which often undesirably exists in combustion exhaust gas, from the exhaust gas.
For example, in a typical wet-type method, the combustion exhaust gas is contacted with an alkali sulfite-containing aqueous solution, whereby sulfur dioxide contained in the exhaust gas is absorbed in the aqueous solution to react with the alkali sulfite, producing alkali bisulfite. Slaked lime (calcium hydroxide) or calcium carbonate is added to the resultant aqueous alkali bisulfite solution to form calcium sulfite, thereby removing the sulfur dioxide in the form of calcium sulfite from the combustion exhaust gas. Furthermore, the thus remove calcium sulfite may be oxidized for producing calcium sulfate, i.e., the sulfur dioxide is ultimately removed in the form of calcium sulfate from the combustion exhaust gas. The reaction mechanisms of the wet-type method are represented by the following formulae (1), (2) and (3) where sodium sulfite is used as the alkali sulfite, with oxidation of calcium sulfite according to formula (4). EQU Na.sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O.fwdarw.2NaHSO.sub.3 ( 1) EQU 2NaHSO.sub.3 +CaCO.sub.3 .fwdarw.CaSO.sub.3 +Na.sub.2 SO.sub.3 +CO.sub.2 +H.sub.2 O (2) EQU 2NaHSO.sub.3 +Ca(OH).sub.2 .fwdarw.CaSO.sub.3 +Na.sub.2 SO.sub.3 +2H.sub.2 O (3) EQU CaSO.sub.3 +1/20.sub.2 .fwdarw.CaSO.sub.4 ( 4)
However, when the combustion exhaust gas is contacted with the aqueous alkali sulfite solution for reaction of sulfur dioxide with the alkali sulfite in accordance with the wet-type method, part of the alkali sulfite is inevitably oxidized into an alkali sulfate by means of oxygen which is also contained in the exhaust gas.
When such alkali suflate accumulates to a significant degree in the alkali sulfite-containing aqueous solution, the quantity of alkali sulfite in the solution, available for reaction with sulfur dioxide, decreases to reduce the rate of sulfur dioxide absorption and efficiency. The alkali sulfate, which accumulates as a by-product, is only slightly reactive with slaked lime or calcium carbonate which is added to the alkali bisulfite-containing aqueous solution for the purpose of forming calcium sulfite. The alkali sulfate thus produced must be removed in advance from the alkali bisulfite solution by a suitable method.
Furthermore, there is known another method of removing sulfur dioxide in the form of calcium sulfite from combustion exhaust gas, wherein a calcium compound such as calcium hydroxide, calcium carbonate or the like is employed in the form of a slurry. In this method, sulfur dioxide can be eliminated in the form of calcium sulfite from combustion exhaust gas in accordance with the following formula (5). EQU Ca(OH).sub.2 +SO.sub.2 .fwdarw.CaSO.sub.3 H.sub.2 O (5)
The resultant calcium sulfite is converted by oxidation into calcium sulfate in accordance with the afore-mentioned reaction formula (4). However, this method is disadvantageous in that, when calcium carbonate is used as a calcium compound, the efficiency of collecting sulfur dioxide from the combustion exhaust gas is reduced since calcium carbonate exhibits extremely low solubility in water and the velocity of reaction of calcium carbonate with sulfur dioxide is lower than that of a water-soluble alkali sulfite. This method is also disadvantageous in that, when calcium hydroxide, which has a relatively high solubility in water, is employed as a calcium compound, the efficiency of collecting sulfur dioxide is similarly reduced since calcium hydroxide is easily converted into calcium carbonate having lower solubility in water by reaction with carbon dioxide which generally exists in combustion exhaust gas in an amount far greater than sulfur dioxide. In order to overcome the above disadvantages, there has been proposed, for example, in U.S. Pat. No. 3,632,306, a method of adding a weak acid such as acetic acid to the slurry of a calcium compound to produce a calcium salt of a weak acid for increasing the concentration of calcium ions in the slurry.
The method of the above U.S. Pat. appears theoretically reasonable at first sight since a part of the calcium carbonate or calcium hydroxide, which is insoluble or sparingly soluble in water, is formed into a water-soluble calcium salt by means of a small amount of a weak acid and then sulfur dioxide in the exhaust gas is collected by means of the resultant calcium salt containing aqueous solution. However our experiments revealed that the efficiency of absorbing sulfur dioxide from the exhaust gas in accordance with the method of the above U.S. patent is yet insufficient in comparison with a method using a solution of a water-soluble salt such as alkali sulfite (when determined by the use of the same apparatus). This is considered due to the following reasons.
For example, when acetic acid is used as a weak acid, sulfur dioxide is absorbed in the solution by the following reaction formulae (6) and (7) EQU CaCO.sub.3 +2CH.sub.3 COOH.fwdarw.(CH.sub.3 COO).sub.2 Ca+CO.sub.2 +H.sub.2 O (6) EQU (CH.sub.3 COO).sub.2 Ca+SO.sub.2 +H.sub.2 O.fwdarw.CaSO.sub.3 +2CH.sub.3 COOH (7)
In order to absorb sulfur dioxide contained in the combustion exhaust gas into an absorbing solution in accordance with the formula (7), a sufficient amount of calcium acetate is essentially required to exist in the solution. However, the reaction velocity for producing calcium acetate according to formula (6) is considered relatively slow because the acetic acid produced in accordance with formula (7) is moved through the solution by diffusion from a point at which the sulfur dioxide is contacted with calcium acetate to a point at which calcium carbonate exists in the solution to react with calcium carbonate in situ and then the resultant calcium acetate is moved by diffusion to the point for contact with sulfur dioxide. In general, a weak acid such as acetic acid reacts at a relatively low velocity with a solid compound such as calcium carbonate. Moreover, part of sulfurous acid produced by dissolution of sulfur dioxide into the solution is moved onto surfaces of calcium carbonate particles, due to the slow production velocity of calcium acetate as previously mentioned, to react therewith for production of calcium sulfite. The calcium sulfite thus produced is insoluble in water and covers the surfaces of the particles. Accordingly, the production velocity of calcium acetate will be further lowered due to reduction in surface areas of calcium carbonate particles capable of contacting with acetic acid. The calcium sulfite thus produced has an extremely small particle size. When the calcium sulfite is oxidized into calcium sulfate, the resultant calcium sulfate has also an extremely small particle size. In this connection, it is generally difficult to separate calcium sulfite or calcium sulfate from the solution by filtration under sufficient washing. Therefore, the method of the above U.S. Patent has disadvantages resulting from the use of a calcium compound in the form of a slurry in the reaction system i.e., efficiencies in operation, maintenance and control of the reaction apparatus are lowered and the slurry of a calcium compound tends to adhere to inner walls of the reaction apparatus, producing so-called "scales".
It is, accordingly, desirable to effectively remove sulfur dioxide from combustion exhaust gas in a simple procedure without reducing the efficiency in removing sulfur dioxide from combustion exhaust gas and without forming scales on inner walls of a reactor.